This invention relates to a light-sensitive, direct positive silver halide photographic material, more particularly to a light-sensitive silver halide photographic material of internal latent image type containing a novel, development-accelerating compound, as used for the formation of a direct positive image by imagewise exposure followed by exposing overall to light before and/or during development, or developing in the presence of a fogging agent.
The present invention provides an art which can be applied also to color diffusion transfer besides ordinary black-and-white photography and color photography. It is well known that a direct positive photographic image can be formed by using a light-sensitive silver halide photograhic material but without requiring any intermediate treatment step or formation of a negative photographic image.
Known methods which have heretofore been employed in order to form a positive image by using a light-sensitive, direct positive type, silver halide photographic material can be classified, excepting specific ones, primarily into two types, in view of the practical usefulness.
According to one of the above two types, a silver halide emulsion which has previously been fogged is used and fog nucleus (latent imge) at the exposed portion is destroyed by the utilization of a solarization or Herschel effect to yield a positive image after development.
According to the another type, a silver halide emulsion of internal latent image type, which has not been fogged is used and, subsequent to imagewise exposure, surface development is effected after and/or during fogging treatment to yield a positive image. In the present specification the surface development effected after and/or during the fogging treatment is referred to as "fogging development" hereinafter. The above fogging treatment may be effected by applying overall exposure to light, by using a fogging agent, by using a strong developer or by effecting heat treatment, but ordinarily a method comprising utilization of light or a fogging agent is adopted. Further, the silver halide photographic emulsion of internal latent image type means such silver halide photographic emulsion that possesses a sensitive speck primarily within the silver halide grains and forms a latent image within said grains by exposure.
In comparison with the former type method, the latter type method is generally higher in the sensitivity and thus suitable for an application where high sensitivity is required. This invention relates in particular to this latter type method.
In the field of this art, a variety of arts have been known heretofore. Those which are principal can be seen in, for example, U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,761,266, 3,761,276 and 3,796,577 and British Pat. No. 1,151,363. According to these known processes, it is possible to prepare a light-sensitive photographic material having a relatively high sensitivity for a direct positive type.
With regard to a detail of the mechanism of forming a direct positive image, it cannot be said that definite explanation has been given but, for example, referring to "Internal-Image Desensitization" as discussed in "The Theory of the Photographic Process", by C.E.K. Mees and T. H. James, third edition, page 161, the course of the formation of a positive image can be understood to certain degree. Thus, it is considered that by virtue of the surface desensitization due to the so-called internal latent image which is formed within silver halide grains by the first imagewise exposure, fog nucleus is selectively formed only on the surface of unexposed silver halide grains and then a photographic image is formed in the unexposed portion by surface development.
In order to selectively nucleate as above, there has been known to give uniform light-exposure over the whole surface or to use as the fogging agent a hydrazine compound or a heterocyclic quaternary nitrogen salt compound.
However, in the application of these methods where a direct positive image is formed by the use of light or a fogging agent to a variety of photographic fields, there are retained further technical problems which should be improved. For example, in such fogging development, fogging by virtue of light or a fogging agent must be preceded prior to a development reaction in order to nucleate fog nucleus, which induce the development reaction and consequently the induction period until the time when development reaction is initiated is longer in comparison with the ordinary development of latent image silver, and as the result, the development rate is considerably delayed. Accordingly, when applied, in particular, to light-sensitive multi-layer color photographic material, there are caused such problems that unevenness in the sensitometry characteristics is liable to be caused between layers. Furthermore, the finally obtainable maximum density is relatively low.
As a method for increasing the maximum density in the color diffusion transfer process using a direct positive silver halide emulsion, use of a hydroquinone sulfonic acid derivative is described in U.S. Pat. No. 3,227,552. However, in the case where such compound is used together, the following disadvantage has been found to be caused. When a positive image is formed by using the above-mentioned hydroquinone sulfonic acid derivative together with a fogging agent, it was found that the background portion was outstandingly discolored to blue and stained, although the maximum density of the image finally obtained was increased. Such staining not only damages the background portion of a photographic image but also particularly degrades the quality of color image when applied to color photography and consequently such problem becomes one which may be said disadvantageous.